(a) Field of the Invention
The present invention is related to an antenna for the reception of MF (Medium Frequency)-band radio waves, and an input circuit for such antenna suitable for a radio receiver.
(b) Description of the Prior Art
In the MF-band, radio broadcasting is conducted usually on an AM (amplitude-modulation) system, and thus radio broadcasting tends to be more susceptible to interference by noise radiations from various external noise sources than is FM (Frequency-Modulation) broadcasting in the VHF (Very-High-Frequency)-band.
In the MF-band, generally, the interfering noise radiations may be considered to have three kinds of forms including electromagnetic wave, magnetic field and electric field. In a usual condition to use a radio receiver, the noise radiation in the form of electromagnetic wave applied to the receiver is at a sufficiently low level of magnitude as compared with the radiations in the other two kinds of forms. Radiations in the form of magnetic field and electric field, on the other hand, are at a relatively high level of magnitude as, for example, in MF-band. However, radiation in the form of magnetic field rapidly attenuates during propagation in proportion to the third-power of the propagation distance, so that such radiation usually would not cause a serious problem for a receiver installed at a site relatively remote from the noise source. Therefore, it is the radiation in the form of electric field that is most problematical and requires solution in, for example, MF-band radio reception, since the level of magnitude of this radiation will decrease only in proportion to the second-power of the propagation distance from the noise source.
For the reasons stated above, magnetic-field-sensitive type antenna are advantageously used for the reception of MF-band radio ways. In fact, it has been the common practice that MF-band radio receivers employ a ferrite-bar antenna which has been considered to be sensitive, in principle, to magnetic fields only. Nevertheless, such radio receivers are more or less interfered by noise radiations of electric field for the reasons as will be described below.
A known typical ferrite-bar antenna and its connection to a radio receiver are schematically depicted in FIG. 1, in which the antenna, generally indicated at 1, includes a bar or stick member 2 made of a ferrite material, and primary and secondary windings 3 and 4 formed on a bobbin 5 which is slidably or non-slidably mounted on the bar member 2. The primary winding 3 is operative as a tuning coil, and is connected to a variable capacitor, i.e. a tuning capacitor 6, in the receiver to form a tuning circuit. Received and tuned radio signals are delivered from the secondary winding 4 to a front end circuit (not shown) of the receiver. The inductance of the primary winding 3 is usually about 300 microhenries or more, so that the ferrite-bar antenna will naturally entail a high-impedance portion, of which an impedance reaches one or several hundreds of kilo-ohms over MF-band frequencies. Such a high-impedance portion of the antenna including the primary winding 3 and often including a certain length of the antenna leads naturally is required to be installed outside the radio circuitry which is shielded from static electricity. Therefore, such portion of antenna inconveniently tends to catch noise radiations which are in the form of electric field. As such, the conventional ferrite-bar antenna is relatively susceptible, against expectation, to interference by noise radiations of electric field.
It should also be understood that such high-impedance antenna, in turn, will limit the extension of the antenna leads from the antenna and the receiver. If the antenna leads are extended for a substantial distance, the possibility of interference by noise electric-field radiations will increase accordingly, and besides an increased stray capacitance on the antenna leads would impose an adverse effect on the operation of the tuning circuit of the receiver. In the past, therefore, conventional ferrite-bar antenna has to be installed close to the receiver, so that the antenna leads may be minimized of their length.
A popular antenna installation often adopted in AM radio receivers is schematically shown in FIG. 2, in which a ferrite-bar antenna 1 is swingably supported by supporting means 8 on the rear panel of receiver 7, with the antenna leads 9 being drawn into the receiver so as to run a possible shortest distance from the antenna. This arrangement provides the convenience that the direction of the antenna 1 can be adjusted at will for achieving an optimum receiving condition. On the other hand, however, such arrangement requires a relatively large space for allowing the swinging movement of the antenna bar 1 on the rearside of the receiver. Although the necessity for such large space may not cause much inconvenience in the case of portable-type radio receivers, this type of requirement quite often fails to be met in case of console-type radio receivers because they are usually installed close to a room wall, a corner of a room, or by the side of furniture, in such a way that the rear panel of the receiver faces the wall or the furniture. In such console-type radio receiver, it would certainly be convenient if an antenna can be installed at any arbitrary desired location away from the rear panel or the receiver per se. However, such arrangement of location of the antenna would, in most cases, require extended antenna leads or wires for the connection of the antenna to the receiver.